1. Field of the Invention
The present invention generally relates to an OPC server redirection manager, which is an intermediate OPC server that forwards all OPC calls made by clients to actual OPC servers that communicate with data sources. In particular, the OPC server redirection manager maintains the information necessary to take appropriate action when a target OPC server fails and ensures that no bad status is returned to a client.
2. Description of the Related Art
OLE FOR PROCESS CONTROL® (OPC®) is an emerging industry standard that facilitates integration by allowing automation systems to share information and interoperate with other industrial automation, process control, and other business systems for plants or factories. (The letters O-P-C originally stood for Object Linking and Embedding (OLE) for Process Control, but OLE has been restructured from object-oriented to object-based and renamed ActiveX.) The OPC standard is a non-proprietary technical specification that is maintained by the OPC Foundation®.
In the past, each software or application developer needed to write a custom interface (or server/driver) to exchange data with hardware field devices. The OPC standard replaced this practice by providing a common interface that permits this work to be done once, and then easily reused by Human Machine Interface (HMI), Supervisory Control and Data Acquisition (SCADA) control and custom applications.
The OPC specification is based upon the Microsoft® OLE (now ActiveX), Component Object Model (COM), and Distributed Component Object Model (DCOM) technologies. OPC defines a standard set of interfaces, properties, and methods for use in process control, manufacturing, and automation applications. These applications include distributed control systems, programmable logic controllers, input/output (IO) systems, smart field devices, and other servers of real-time information. The ActiveX/COM technologies define how individual software components interact and share data. By using the Microsoft® Visual Basic object-oriented component, OLE Automation, OPC can provide office applications with plant floor data via local area networks (LANs), remote sites, or the Internet.
OPC provides many benefits to end users and hardware and software manufacturers, including open connectivity, high performance, and improved vendor productivity. OPC provides open connectivity giving users a choice among a wider variety of plant floor devices and client software, allowing better utilization of best-in-breed applications. Because OPC is an open standard, software and hardware manufacturers devote less time to connectivity issues and more time to application issues, eliminating significant duplicate effort. OPC fosters greater interoperability between automation and control applications, field devices, and business and office applications.
Some redirection systems provide inferior functionality to the redirection manager according to the present invention and they appear to have different architectures. One redirection system is implemented as a Microsoft Windows® service, whereas the redirection manager according to the present invention uses a private connection approach to limit the impact of failure. Redirection systems implement OPC data access, whereas the redirection manager according to the present invention has redirection servers that implement those as well as OPC alarms and events. Redirection systems appear to implement server/node health via a ping mechanism, whereas the redirection manager according to the present invention uses a monitoring approach with less network traffic overhead. Certain OPC servers synchronize the internal OPC server database for performance and consistency reasons. The redirection manager according to the present invention optionally provides this capability.
Other redirection systems provide a service to which all clients connect. If that service fails, then all connected clients are impacted. By contrast, clients using the redirection manager according to the present invention have a private connection. If the private connection fails, then only the connected client is impacted. Conventional redirection systems use a ping mechanism for detection. This is less efficient than the redirection manager according to the present invention, which uses notification providers to alert the redirection manager when target nodes or servers fail.
The present invention overcomes these problems by maintaining the information necessary for taking appropriate action when the target OPC server fails and for ensuring that no bad status is returned to the client. Any existing OPC client needing seamless failover between target OPC data access and/or alarm and event servers will connect to a uniquely configured instance of the redirection manager instead of connecting to the primary server. The client application continues to perform OPC related tasks as usual. Because each client connection is unique, the redirection manager itself is not a single point of failure. Each configured redirection manager instance monitors the health of the target server node and the target server itself by subscribing to failure events. Use of subscription rather than polling (via ping) reduces network traffic. The present invention also provides many additional advantages as described below.